1. Field of the Invention
The invention relates to new photosensitizing transition metal complex and its use for photovoltaic cell such as solar cell.
2. Description of the Related Art
Photosensitive dyes are coated on metal oxide films rendering a device as solar cell effective in the conversion of visible light to electric energy. In this solar cell, a monolayer of dye is attached to the surface of nanocrystalline metal dioxide film. Photoexcitation of the dye results in the injection of an electron into the conduction band of the metal oxide. The original state of the dye is subsequently restored by electron donation from a redox system, such as iodide/triiodide couple. Molecular design of ruthenium polypyridyl photosensitizers for nanocrystalline TiO2 film in solar cell that can absorb visible lights of all colors presents a challenging task. The dyes should have suitable ground- and excited state redox properties so that the two key electron transfer steps (charge injection and regeneration of the dye) occur efficiently.
The most efficient transition metal complexes employed so far in the solar cell are Ru(II) polypyridyl complexes because of their intense charge-transfer (CT) absorption in the whole visible range, moderately intense emission with fairly long lifetime in fluid solution at ambient temperature, high quantum yield for the formation of the lowest CT excited state, and redox reactivity and ease of tunability of redox properties. So far, the most successful photosensitizers employed in solar cell are Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2 and Ru(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)(NCS)3. The role of the monodentate thiocyanato ligands is to tune the spectral and redox properties of the photosensitizers by destabilization of the metal t2g orbital.
The presence of monodentate donor ligands (NCS—) can undergo ligand photosubstitution or photodegradation reaction via population of an upper lying ligand field excited state and these processes can be reduced by multidentate ligands.
As relevant prior arts are mentioned U.S. Pat. No. 6,245,988, U.S. Pat. No. 5,789,592, Japanese Patent Kokai No. 2003-212851 and New J. Chem. 26 (2002) 966-968.